1. Field of the Invention
The present invention relates to a process for producing hematite for ironmaking.
2. Description of the Related Art
Particularly, the present invention relates to a technique for recovering hematite for ironmaking from a leach residue in a slurry state (hereinafter referred to as a tailings slurry) obtained from a final neutralization step in a hydrometallurgical plant by a high pressure acid leach (HPAL) process for nickel oxide ore.
Nickel has been widely used as a raw material of stainless steel, but with a tendency of depletion of a sulfide ore as a raw material of nickel, a technique of refining low-grade oxide ore has been developed and put into practical use.
Specifically, a production process called a high pressure acid leach (hereinafter referred to as HPAL) process has been put into practical use, in which a nickel oxide ore such as limonite and saprolite is put in a pressure device such as an autoclave together with a sulfuric acid solution, and nickel is leached at a high temperature of about 240 to 300° C. and high pressure.
FIG. 3 shows a schematic flow chart of the production steps of the HALP process.
Nickel leached in a sulfuric acid solution in the HPAL process is separated from a leach residue by adding a neutralizing agent to neutralize excess acid, followed by solid-liquid separation.
Subsequently, the nickel is subjected to a step of separating impurities and then recovered as an intermediate material in the form of a hydroxide, a sulfide, or the like, and the intermediate material is further refined to thereby obtain nickel as nickel metal or nickel chloride.
Note that in the step of neutralizing excess acid, the leachate is adjusted to a pH suitable for solid-liquid separation, and in the next step of solid-liquid separation, the concentration of solids and solid-liquid separation are performed in a facility called CCD (Counter Current Decantation). Generally, a plurality of successive stages of thickeners are used in CCD.
A liquid component (hereinafter may be referred to as an overflow) obtained from the CCD is transferred to a neutralization step so as to be adjusted to a pH suitable for a sulfurization step. In the neutralization step, the pH is adjusted, and fine solids produced are removed by precipitation. Then, the resulting liquid component is, for example, subjected to sulfurization treatment to produce an intermediate material, nickel sulfide.
In such a HPAL process, nickel can be leached substantially completely, for example, in the case of nickel oxide ore, even if the ore is a low grade ore containing a target valuable metal to be recovered in an amount of 1 to 2% by weight or less (hereinafter, % by weight is represented by %). That is, a target metal can be obtained by concentrating a target metal to the same degree as in a conventional raw material followed by employing substantially the same refining process and steps. Further, the HPAL process can be applied not only to nickel oxide ore but to other raw materials, such as nickel sulfide ore, copper sulfide ore, and copper oxide ore.
On the other hand, the main component of the leach residue remaining after nickel is recovered in the HPAL process is iron oxide; the content of iron in the solids of the leach residue is about 40 to 50%, and the quantity of production of the leach residue is about 50 times to 100 times of the quantity of production of the intermediate material. This is because nickel oxide ore and nickel sulfide ore as raw materials contain iron in an amount far exceeding the content of nickel.
The leach residue is in the state of a chemically and environmentally stable oxide since it is produced at high temperatures, but under the present circumstances, it does not have particular utility value and is accumulated and stored in a residue stacking yard.
Therefore, a large residue accumulation yard is required for accumulating and storing a huge amount of a leach residue produced with the operation of the HPAL process.
Incidentally, iron ore, which contains a large amount of iron oxide, is widely utilized as a raw material for steelmaking. In steel smelting, the iron ore containing iron oxide is charged into a blast furnace together with a reducing agent such as coke and heated to reduce and melt the iron ore to obtain crude steel. The crude steel is refined in a converter to produce target steel.
Generally, the iron ore serving as the raw material is limited resources, and, moreover, it is increasingly difficult to acquire good quality iron ore required for maintaining the quality of steel. Therefore, investigation of using a leach residue as iron ore has been made.
However, it has been difficult to directly use the leach residue from the HPAL process as a raw material for ironmaking for the following reasons.
Since the leach residue from the HPAL process contains vein stone, impurities, particularly sulfur in addition to iron oxide, it was not suitable as a raw material used for conventional common ironmaking processes. Specifically, this is because the leach residue has high sulfur grade.
Particularly, the sulfur grade in the iron oxide that can be utilized as a raw material for ironmaking, which changes depending on the equipment capacity, quantity of production, and the like of each iron mill, is generally required to be suppressed to less than 1%.
However, the solids of the leach residue contain about 5 to 8% of sulfur. Most sulfur in the leach residue is derived from calcium sulfate (gypsum) that is mixed in the HPAL process.
When free sulfuric acid (free sulfuric acid means an unreacted remaining acid in the sulfuric acid excessively added to perform sufficient leaching in the HPAL process) remaining in the leached slurry obtained in high pressure acid leach is neutralized, a neutralizing agent that is common, inexpensive, and calcium-based, for example limestone or slaked lime, is added. The gypsum is produced by the reaction between the calcium contained in the neutralizing agent and the free sulfuric acid and mixed into the leach residue.
Note that a part of sulfur (about 1%) contained in the leach residue solids is incorporated into hematite particles produced.
The solids in the residue after nickel leaching obtained at this time comprise particles which mainly comprise hematite and have a particle size of about 1 μm, and the iron grade in the solids is about 30 to 40%, and the sulfur grade is about 5 to 8%. Note that the moisture content of the leach residue obtained at this time is 60%.
In order to use this leach residue as hematite for ironmaking, it is necessary to refine the iron grade in the leach residue solids to 50% or more and to refine the sulfur grade to 1% or less.
As the technique for refining a leach residue, for example, Japanese Patent Laid-Open No. 2010-095788 describes a technique of subjecting the leach residue to separation by sieving, separation by a wet cyclone, or magnetic separation to remove impurities in a hematite mixture, and a certain effect has been observed in order to remove impurities in hematite.
However, the hematite obtained by the invention disclosed in Japanese Patent Laid-Open No. 2010-095788 has not been satisfactory when it is used singly as hematite for ironmaking. Particularly, the highest iron grade in the hematite obtained by the invention disclosed in Japanese Patent Laid-Open No. 2010-095788 is about 40 to 45%. Therefore, it has been necessary to mix it with a raw material for ironmaking containing a higher grade of iron in order to use it as hematite for ironmaking. Note that the moisture content of the leach residue obtained after physical separation in Japanese Patent Laid-Open No. 2010-095788 is about 40%.
The present invention, which has been made in order to provide a solution to such circumstances, proposes a process for separating a leach residue from which a hematite-containing material that can be used as a raw material for ironmaking can be obtained, and provides a production process for producing hematite for ironmaking from the leach residue.